Alumina has long been the major raw material for the production of fine ceramics, especially in the fabrication of the electrical circuit substrates. High-purity (purity &gt;99%) and fine-grained alumina ceramics have recently been developed to improve the tensile strength and abrasion resistance thereof. The manufacturing process of these high-purity and fine-grained alumina ceramics generally comprises the following steps:
1 ) preparing an aqueous dispersion of fine-sized alumina particles; PA1 2) casting the aqueous dispersion to form a green body; and PA1 3) sintering the green body.
Recent study shows that the colloidal processing can effectively enhance the strength of fine ceramics. {H. Mizuta et al., J. Am. Ceram. Soc., 75[2]pp. 469-473 (1992); N. McN. Alford et al., Nature, Vol. 330[5]pp. 51-53 (1987)} Further, green bodies of complicated shapes can be formed using various casting techniques such as slip casting and pressure casting, wherein uniformly distributed sub-micrometer alumina particles having no substantial agglomerates are used to prepare an aqueous colloidal suspension (or termed "slurry"), and the aqueous colloidal dispersion is then casted to form a relatively dense green body. The green body is dried to an appropriate extent and then sintered into a ceramic product having a homogeneous microstructure of average grain size ranging 1-10 .mu.m, whereby the ceramic product have a better mechanical strength.
Up to the present, none of the existing processes that involve colloidal processing of agglomerate-free sub-micrometer alumina particles use transition phase alumina particles. Moreover, in order to obtain the uniformly distributed and agglomerate-free sub-micrometer alumina particles a sedimentation or centrifugal casting process are required to eliminate large particles and avoid agglomeration, and these in turn cause a loss of the high purity alumina particles.
Because their specific surface area is greater than 15 m.sup.2 /g, the above mentioned uniformly distributed and sub-micrometer alumina particles require a very powerful dispersion agent to ensure that a high solid content aqueous colloidal slurry of the sub-micrometer alumina particles can be prepared. Generally, there are two types of dispersing agents. The first type of dispersing agent can make the surface of a particle charged and the Coulombic forces thereby generated between the particles can ensure an effective dispersion. The second type of dispersing agent generates steric hindrance between particles to achieve effective dispersion.
As disclosed in U.S. Pat. No. 5,246,624, an electrostatically stabilized dispersion of alumina particles is usually achieved by controlling the pH value of the slurry. However, this method of controlling pH value is less than ideal when dealing with alumina particles having a large specific surface. {For further information in this regard, please refer to Cesarano and Aksay, J. Am. Ceram. Soc., 71[4]250-255 (1988)} Alumina particles with a large specific surface tend to neutralize acidic or basic conditions when they are well mixed in an aqueous solvent medium. In neutral condition, the alumina particles slurry has a tendency to turn into gel because hydrogen bonds will be formed between the surfaces of the sub-micrometer alumina particles and thus cause a bridging effect. The gelling of the slurry could cause a problem as air bubbles generated during mixing are more likely to remain in the slurry because of its increased viscosity.
The second type polymer based dispersion agent, especially high molecular weight polymer, could jeopardize the process of manufacturing ceramics because of the complicated nature of how they are adsorbed to and desorbed from the surface of the alumina particles, and how they affect the viscosity of the slurry.
The primary objective of the present invention is to provide an aqueous colloidal dispersion of sub-micrometer alumina particles for manufacturing high-density and fine-grained ceramics.
Another objective of the present invention is to provide a new electrolyte based dispersing agent for preparing a high solid content and sedimentation-free aqueous dispersion of sub-micrometer alumina particles, in which transition phase sub-micrometer alumina particles can be used for the preparation.